This invention relates to improvements in joining ligatures, such as straps and wires, and is particularly concerned with a seal and a method for applying a seal to form a joint between overlapped portions of a ligature to form a secure joint therebetween and to prevent relative movement of the overlapped portions of the ligature.
Ligatures of the type with which the present invention is concerned are used in many diverse fields. Their most common use, however, is in the field of packaging and securing. In the usual packaging situation, the package or bundle is encircled with a ligature until the ligature forms a loop with a portion of the ligature overlapping upon itself. The leading end segment of the ligature is then gripped and the trailing portion of the ligature is tensioned or pulled to tightly engage the ligature loop about the package. After sufficient tension has been pulled, the loop is secured, as by connecting the overlapping portions of the ligature together.
Many forms of ligatures of the types under consideration are in common use. The most common forms are wire and strap made of plastic, aluminum, iron and steel. The present invention has general utility with respect to all of the aforementioned types of ligatures.
One widely used method of connecting the overlapping ligature segments together about a package is to apply a separate, relatively small clamp around the overlapping ligature segments. The clamp can have many forms but is most commonly a piece of sheet steel stock which has a generally flattened, C-shaped configuration and which is known in the industry as a seal or seal blank. The seal is placed around the overlapping ligature segments and compressed thereabout with an appropriate tool (either manual or automatic and either portable or stationary). Typically the compression includes the formation of one or more notches or crimps within the seal blank, which crimps comprise regions of relatively high contact pressure between the seal and the overlapping ligature segments.
Different types of ligatures and seals have been developed over the years. However, it has been found that the joint formed with many of the types of ligatures and seals is not entirely satisfactory. Specifically, the joint may not be initially tight enough or the joint may loosen with time and/or upon being subjected to external loading conditions such as vibrations or impact during handling.
Many of the types of ligatures in use today are specially treated or coated with paint, wax, grease, oil, or other materials to prevent corrosion, improve appearance, improve automatic feeding characteristics within automatic strapping machines, or for other reasons. Some straps may also be especially heat treated. In any case, such treated straps may have a lower coefficient of sliding friction than untreated strap and may slip more easily in a joint formed by a crimped or compressed seal.
Also, in many industrial packaging situations, oil or grease may be accidentally or purposely applied to the strap. In any case, application of a compressed seal about such strap segments to effect a friction joint therebetween may not be performed with sufficient force to establish a tight enough joint that will hold under the tension applied to the strap. Even if the seal at first securely grips the overlapping strap segments, the strap segments may start to slide within the seal over a period of time or when subjected to vibration or other shock loading conditions.
With plastic strap the strap surface may be relatively smooth and have a relatively low coefficient of sliding friction. In addition, plastic strap has a tendency to stretch and undergo a transverse reduction in the width dimension when subjected to substantial tensile forces over a period of time. Obviously, these characteristics can decrease the joint strength capability or integrity of a joint formed with a compressed seal at given compression force.
To overcome these problems of seal/ligature slippage, a number of seal modifications have been developed. The U.S. Pat. No. 3,089,233 to Meier discloses a seal blank which is coated on the inside with relatively hard, small grit particles. When overlapped end portions of a strap are secured together, the particles are embedded in the adjacent faces of the strap ends and hold the strap ends against relative longitudinal movement. Similarly, the U.S. Pat. No. 3,237,256 to Young discloses a seal for plastic strap with grit material secured to the inner, strap-contacting surfaces of the seal.
Though the above-discussed grit-type seals function satisfactorily to cut through layers of wax, oil, or paint on strapping and form a secure joint, the grit on the seals poses a problem since some of the particles of grit tend to become detached from the seal and are then carried, or fall, into the tool or machine used to compress the seal about the overlapped straps. Eventually, a build-up of grit within the tool or machine causes operational problems. Thus, it would be desirable to provide a gritless seal free of any sources of particulate matter which could enter a strapping machine or seal applying machine or tool and have deleterious effects.
Other types of seals have been developed for providing increased gripping capacity and which do not use grit. Examples of such seals are those employing specially configured gripping projections or protuberances on the inner surface of the seal such as the seals for plastic strap disclosed in the U.S. Pat. No. 3,197,831 to Martin et al. and the U.S. Pat. No. 3,636,592 to Beach.
Though gritless seals of the above-described type have been found to function satisfactorily, they are not without disadvantages. The protuberances on the inner surface of the seal must be especially formed within the seal. This, of course, involves metal working operations such as stamping.
Personnel safety hazards and shipping problems also arise from the use of such gritless seals. Since the seals are shipped with sharp projections formed therein, the user of the seal must be careful not to cut himself on the projections during any handling operations involving the seal, as when loading the seals into a seal applying machine or into an automatic strapping machine. Further, the projections increase the thickness of the seal and thus decrease the number of seals that can be nestably stacked together within a given size shipping container.
Consequently, it would be desirable to provide a seal which would not require the additional metal working steps of forming special gripping projections and which would not have sharp projections which could injure the user. Further, it would be desirable to provide a seal which could be used on all types of strap and which could be supplied to the user in the form of a seal blank having relatively flat surfaces which would allow closer nestable stacking to permit a greater number of such seals to be packed within a given size box.